Hi,
I am in the same position at the moment with a mum and 7 kittens.
The best way to alleviate some of the stress is to rig up some curtains halfway across the pen so that you can go in and do what needs doing. I also cover the whole pen front as well for long periods so that mum cat can come out away from the kittens and feel safe.
She still lunges as me but its mainly posturing and I can ignore most of it though try not to jump - which is instinctual when a mummy cat lunges and spits 12 inches from your face!
Feliway may also help ( I Keep forgetting) - if you have some spray all covers liberally.
If she is still too scary then use a bit of cardboard to hold against the curtain as a shield so that if she does a full lunge you are more confident that you are safe.
I am hand feeding mum cat roast chicken which she likes and will snatch off me with a snort. I just have to be careful how I retract my hand as I think once she thought my finger was chicken and that I was trying to take it away from her.
Also lots of talking in a low friendly voice as you approach the pen and do your business. No direct staring at her or the babies - she will see that as a threat so lots of side glances and slow blinkng. Slow movements and no sudden ones if you can help it
I also close the door to the room when i'm doing , not that I think she would lunge past me but to stop other cats wandering around and upsetting her and me being in the line of fire.
I don't know if this is the 'professional' approach but so far I am unscathed (I shouldn't have said that should I
).
If i lift one corner of the cutain I can change the food bowl and the other gives me access to the water bowl in her inner sanctdom (sp).
Can you see her head resting against the tray, I think she's on the look out for breakfast ... talking of which , duty calls!